In wireless communication systems employing LTE or WiMax, Hybrid Automatic Repeat Request (Hybrid ARQ, or brief HARQ) is applied to sustain link quality between transmitter and receiver even under bad channel conditions.
In Hybrid ARQ systems, in case the receiver detects an erroneous reception of a data packet or a decoding error, e.g. by evaluating a cyclic redundancy check (CRC) sum, it indicates this to the transmitter by returning a negative acknowledgement (NACK) message. In this case, the transmitter encodes and modulates the same data packet again and repeats the previous transmission. The receiver demodulates the repeated data packet, combines the newly computed softbits with the softbits from the previous transmission, and starts another decoding attempt. In this way, the probability of erroneous decoding at the receiver is decreased. This interaction of feedback and retransmission can be repeated multiple times, which increases link robustness between transmitter and receiver.
To this end, however, a memory (HARQ memory) is required at the receiver, in order to store the received signal to be combined with the retransmitted signal at a later time. In particular, the received signal is stored after softbit computation. The receiver always has to store the softbits of a complete data packet in the HARQ memory, the HARQ memory's size thereby being determined by the maximum length of the encoded data packet. In LTE which uses multi-antenna arrangements at the transmitter and/or receiver, multiple HARQ processes that include information feedback and retransmission are running in parallel. Since the receiver has to provide memory for every HARQ process, the total HARQ memory size required is increased significantly.
For wireless communication systems that apply the Hybrid ARQ protocol, the HARQ memory is one of the dominant contributors to die size and power consumption. In other words, the HARQ memory size defined for LTE is very significant for embedded receiver designs in terms of both, silicon area and power consumption. In LTE, different receiver categories have been defined which require HARQ memory sizes for up to 3,667,200 softbits.
Therefore, it is very much desirable to shrink the HARQ memory size. Any reduction of the HARQ memory size is desirable for a chipset vendor in order to gain competitiveness.